The Unbearable LightnessofNeutrinosDave Casper University of California, Irvine

D. Casper, University of California Irvine

Lightness and WeightParmenides Query: Which is positive? Lightness or Weight?Today we wonder: where to look for new physics?High energy/mass?Or the lightest particle known the neutrino?Journal Publications: Fraction on neutrinos by year

D. Casper, University of California Irvine

A Desperate Remedy

D. Casper, University of California Irvine

Operation Poltergeist

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Two Kinds of NeutrinosReines and Cowans neutrinos produced in reaction: Observed reaction was: Muon decay was known to involve two neutrinos: If only one kind of neutrino, the rate for the unobserved process: much too largeProposal: Conserved lepton number and two different types of neutrinos(e and )Produce beam with neutrinos from Neutrinos in beam should not produce electrons!

D. Casper, University of California Irvine

Last, but not least

D. Casper, University of California Irvine

Threes CompanyNumber of light neutrinos can be measured!Lifetime (and width) of Z0 vector boson depends on number of neutrino speciesMeasured with high precision at LEPN = 3.02 0.04Probably no more families exist

D. Casper, University of California Irvine

Weighing the NeutrinoMass of e should alter endpoint of -decay spectrumExtremely difficult measurement!m(e) < 2.2 eV (90%)Mass of from -decay:m() < 0.19 MeV (90%)Mass of from -decay:m() < 18.2 MeV (90%)

D. Casper, University of California Irvine

Neutrinos in the(Minimal) Standard ModelThree generationsRight handed neutrinos do not existNeutrinos unable to acquire mass through the Higgs mechanism without right-handed component

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Proton DecayGeneric Prediction of most Grand Unified TheoriesLifetime > 1033 yr!Requires comparable number of protonsColossal DetectorsNeutrino background proved more interesting than the (non-existent) signal

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Water Cerenkov Technique

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Nova

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Atmospheric NeutrinosProducts of hadronic showers in atmosphere2:1 :e ratio from naive flavor countingNormalization uncertainty 20%Flavor ratio (/e) uncertainty 5%

D. Casper, University of California Irvine

The Atmospheric Neutrino ProblemLarge water detectors measure significant deficit of interactionsSmaller detectors do notNeed for larger and more sensitive experiments

D. Casper, University of California Irvine

The Solar Neutrino ProblemHomestake experiment first to measure neutrinos from Sun, finds huge deficit (factor of 3!)Anomaly confirmed by SAGE, GALLEX, Kamiokande experiments

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Neutrino OscillationQuantum mechanical interference effectRequires:Neutrinos have different masses (m20)Neutrino states of definite flavor are mixtures of several masses (and vice-versa) (mixing 0)Simplest expression (2-flavor):Prob(oscillation) = sin2(2) sin2(m2L/E)In three-neutrino case, three angles and two mass differences characterize the behavior

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An Oscillation AnalogyConsider a clock with two hands, A and BThe hands advance with (in principle) different rates (corresponding to m20)We start out with hands at right anglesThe lengths of the hands obeys A2+B2=1The oscillation probability is equal to

D. Casper, University of California Irvine

Summary: 5 years agoNeutrino lives happily within the Standard Model, but some there are some unconfirmed signs of trouble:Solar neutrino problemConfirmed, but no clear indication of new physicsAtmospheric neutrino problemPartially confirmed, but conflicting evidenceLSNDUnconfirmed, conflicting evidenceNew and much more sensitive experiments are coming

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Accelerator, Reactor Searches

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Super-Kamiokande DetectorTotal Mass: 50 ktFiducial Mass: 22.5 ktActive Volume:33.8 m diameter36.2 m heightVeto Region: > 2.5m11,146 50 cm PMTs 1,885 20 cm PMTs

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EvidenceSuperK also sees deficit of interactionsAlso clear angular (L) and energy (E) effectsFinally a smoking gun!All data fits oscillation perfectlySurprise:Maximal mixing between neutrino flavors

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Double ChecksLook for expected East/West modulation of atmospheric fluxDue to earths B fieldIndependent of oscillationFit the data to a function of sin2(LEn)Best fit at ~-1 (L/E)

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Appearance Signal?Best proof of neutrino oscillation: appearanceVery difficult in water detectorFind -like particle ID estimatorLook for excess in upward direction

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The K2K Experiment

D. Casper, University of California Irvine

K2K Results

ObservedExpected (No osc)Expected (310-3 eV2)All Fully Contained2837.83.824.72.71-ring1522.73.213.32.0-like1420.83.211.71.9e-like11.90.41.60.3multi-ring1315.12.511.42.0Outside Fiducial Vol1417.011.0

D. Casper, University of California Irvine

SuperK Solar NeutrinosReal-time measurement allows many tests for signs of oscillation:Day/Night variation Spectral distortions Seasonal variationAllowed oscillation parameter space is shrinkingSMA is disfavored by SK data

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LSND and KARMENLSNDReported signal of e oscillation (e appearance)Possible background problems render result controversialKARMENExperiment with lower backgrounds sees no evidence for effectEntire parameter space not excluded

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Sterile Neutrinos?Too many neutrino oscillation modes!Three neutrinos cannot accommodate m2 values of atmospheric, solar and LSND resultsTwo possibilities:One (or more) experiment is wrong (or not due to oscillation)Additional sterile neutrinos are neededIs another desperate remedy needed?m2m2m2LSND

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Summary: TodayStrong evidence for atmospheric neutrino oscillation from Super-KamiokandeEffect further confirmed by Soudan-II, MACROSolar neutrino problem again confirmed, but still not proven as oscillationLSND and KARMEN still disagreeNeutrinos seem to have mass!New and much more sensitive experiments are coming

D. Casper, University of California Irvine

Beyond the Standard ModelMasses:Disparity between neutrino and charged lepton massesSee-Saw mechanism involves Unification-scale intermediaryMixing:Small in hadronic sectorLarge in at least some of lepton sectorWhat about CP violation?

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2nd Generation LongBaseline (MINOS,CNGS)730 km baselinesMINOS:Factor ~500 more events than K2K (at 3 distance)Disappearance and appearance (e, ) experiments

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SNOWater detector with a difference:Heavy waterAble to measure charged current (e) and neutral current (x)Should determine (finally!) whether solar neutrinos are oscillating or not

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KAMLANDLong-baseline experiment to study reactor neutrino disappearanceUses about 2 dozen reactors spread through-out JapanVery sensitive to LMA solar solutionPossibility to study solar neutrinos directly too.

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MiniBooneMineral oil Cerenkov detector to study LSND effect (e appearance) at FermilabShould avoid backgrounds which plagued LSND

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Summary: 5 year outlookSNO will determine whether solar neutrinos oscillateKAMLAND will find LMA solution if it is the right oneK2K and MINOS will check atmospheric oscillation with accelerator beams appearance may be observedMiniBoone will determine whether LSND result is correctEnd of discovery era?Time for precision measurements

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SuperBeamsSmall m2 (< 1 eV) values appear interestingOscillation probability depends on m2L/ESmall m2 requires either small E or large LSmall E is problematic because neutrino detection probability is proportional to ELarge L is problematic because neutrino flux drops like 1/L2Solution: Increase beam luminosity and/or detector size

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SuperBeam SignaturesStudy of beam designed to look for e appearanceLow beam energy has benefit of reducing backgroundsGood sensitivity even for very small mixing angle

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SuperBeam SensitivityFor this study, with realistic simulation and reconstruction, sensitivity to sin213 down to (few)10-3

D. Casper, University of California Irvine

Neutrino Factory

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Neutrino Factory Signatures30 GeV storage ringBaseline: FNAL/SK (7300 km)450 kton water1020 decays (=1 yr)Look for wrong-sign muons ( appearance)

110K44K52K94K

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Neutrino Factory SensitivityFor this preliminary study, another 1-2 orders of magnitude improvement in sensitivityOther studies are more optimisticCP violation search depends critically on solution to solar neutrino problem (large or small angle mixing)

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World-Wide Neutrino Web?

D. Casper, University of California Irvine

Summary: 10 year outlookPrecision measurement era of neutrino physics underwayExperiments of epic proportions will probably be in the worksA neutrino factory will very likely be built, especially if we get (or stay) lucky:LMA solar solution13 not too small (10-4?)Otherwise SuperBeams may be able to do the job

D. Casper, University of California Irvine

ConclusionsNeutrinos are the final frontier of the Standard ModelAn outpouring of creative energy is now devoted to finding new and more sensitive approaches to taming themMore surprises may lie aheadWhat if LSND is right?What if the solar neutrino problem is other new physics?Major discoveries elsewhere (LHC) during this period?Maybe Parmenides was right?

D. Casper, University of California Irvine

Parmenides positive and negativeHigh energy frontier vs. low mass frontier

Example of crisis of beta decayPaulis proposal

Ideas to detect neutrinos with A-bombIdeas to detect neutrinos with reactorDescription of R&C experimentThe first neutrinosIdea of two neutrinosThe first neutrino beam